A of capacitive measuring devices for making linear and angular measurements have been developed in which a plurality of discrete, capacitively coupled electrodes are displaced relative to each other, thereby causing electrical signals to be produced can be processed to provide interpolated measurement data. Examples of such devices include applicant's own prior device disclosed in U.S. Pat. No. 4,420,754 (the '754 Patent); and the devices disclosed in U.S. Pat. No. 4,586,260 (Baxter et al) U.S. Pat. No. 4,654,524 (Kita).
Various interdependent factors affect the resolution and accuracy of such measuring devices, including the size and spacing of the electrodes, and the precision of the measuring element geometrics and alignment. Also important are the accuracy and of data interpolation, which are a function of the amount of signal noise, including that caused by extraneous coupling between electrodes, and the sophistication of the electrical signal processing.
It has heretofore not been possible to achieve significant improvements in the resolution of capacitive measuring devices without also substantially increasing the cost and complexity of manufacture, introducing misalignment sensitivity or requiring substantial signal processing. For example, in the case of the transducer disclosed in applicant's aforementioned '754 Patent, the resolution is determined primarily by the scale wavelength (i.e., the distance over which the capacitance function for a given signal phase is repetitive, which corresponds to the pitch of the scale electrodes) and the interpolation rate. The shorter is the scale wavelength, the lower can be the interpolation rate to achieve a given resolution. Conversely, increasing the interpolation rate allows a longer scale wavelength to achieve the same resolution.
However, in the transducer disclosed in applicant's '754 Patent, multiple transmitter electrodes, corresponding to the number of phases in the transmitted signal, are disposed within one scale wavelength, and the transmitter electrodes thus must have a markedly smaller pitch than the scale electrodes. Consequently, significantly decreasing the scale wavelength is limited by the ability to substantially decrease the width of and spacing between the respective transmitter electrodes, and requires changing from relatively simple and inexpensive manufacturing technologies, such as printed circuit technology, to complicated and expensive technologies such as thin film vapor deposition technologies. Similarly, increasing the interpolation rate entails progressively more complicated and expensive circuitry, and is particularly difficult to achieve in the case of the low voltage systems employed in portable, hand-held measurement devices such as calipers.
As another example, in the aforementioned Baxter et al system, the transmitter electrodes are divided lengthwise into X and Y groups of electrodes which are in space quadrature. Dual phase high frequency excitation signals are applied to alternate elements of the transmitter electrodes, respectively, and X and Y pickup signals are generated indicative of the signals capacitively coupled from the respective X and Y groups of slider elements to the subjacent receiver electrodes. However, the separate group arrangement of transmitter electrodes causes the Baxter et al system to be extremely sensitive to nonuniformity of the gap between the slider and the scale.
As a further example, the Kita system utilizes two arrays of n and n+m elements, respectively, to produce a set of outputs, the levels of which, when plotted against the successive positions of the elements in one of the arrays, form a wave, the phase of which changes with relative displacement of the two arrays. The displacement is measured by measuring the phase change relative to the pattern of the values of maximum likelihood for the outputs of the elements. However, this measurement technique requires statistical processing of the individual outputs to obtain the position of maximum likelihood of each output, which entails a substantial amount of computations for systems with a large number of array elements.